U.S. patent application number 13/837068 was filed with the patent office on 2014-09-18 for composition for anode in fuel cell.
The applicant listed for this patent is LG FUEL CELL SYSTEMS, INC.. Invention is credited to Richard W. Goettler, Shung-Ik Lee, Zhien Liu.
Application Number | 20140272672 13/837068 |
Document ID | / |
Family ID | 50346190 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140272672 |
Kind Code |
A1 |
Goettler; Richard W. ; et
al. |
September 18, 2014 |
COMPOSITION FOR ANODE IN FUEL CELL
Abstract
In some examples, a fuel cell comprising a cathode; an
electrolyte; and an anode separated from the cathode by the
electrolyte. The active, as-reduced anode includes Ni, La, Sr, Mn,
and O, where the reduced anode includes a Ni phase constitution and
a (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based Ruddlesden-Popper (R-P) phase constitution, wherein n is
greater than zero, and wherein the anode, cathode, and electrolyte
are configured to form an electrochemical cell.
Inventors: |
Goettler; Richard W.;
(Medina, OH) ; Lee; Shung-Ik; (Canton, OH)
; Liu; Zhien; (Canton, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG FUEL CELL SYSTEMS, INC. |
NORTH CANTON |
OH |
US |
|
|
Family ID: |
50346190 |
Appl. No.: |
13/837068 |
Filed: |
March 15, 2013 |
Current U.S.
Class: |
429/527 ;
429/535 |
Current CPC
Class: |
H01M 4/9033 20130101;
H01M 4/8878 20130101; H01M 4/8652 20130101; Y02E 60/50 20130101;
H01M 4/9066 20130101; H01M 4/9058 20130101; H01M 4/905 20130101;
H01M 2008/1293 20130101 |
Class at
Publication: |
429/527 ;
429/535 |
International
Class: |
H01M 4/90 20060101
H01M004/90; H01M 4/88 20060101 H01M004/88 |
Goverment Interests
[0001] This invention was made with Government support under
Assistance Agreement No. DE-FE0000303 awarded by Department of
Energy. The Government has certain rights in this invention.
Claims
1. A fuel cell comprising: a cathode; an electrolyte; and a reduced
anode separated from the cathode by the electrolyte, wherein the
reduced anode includes a Ni phase constitution and a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based Ruddlesden-Popper (R-P) phase constitution, wherein n is
equal to or greater than one, and wherein the anode, cathode, and
electrolyte are configured to form an electrochemical cell.
2. The fuel cell of claim 1, wherein the Ni phase constitution and
the (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having
a Mn-based R-P phase constitution, of n greater than or equal to 1,
is formed via a reduction of a Mn and Ni mixed B-site compound
having a pervoskite structure or Ruddlesden-Popper compound that is
present following an initial anode processing step.
3. The fuel cell of claim 2, wherein the Mn and Ni mixed B-site
compound comprises a
(La.sub.1-xSr.sub.x)(Mn.sub.1-xNi.sub.x)O.sub.3 compound.
4. The fuel cell of claim 3, wherein additional A-site and B-site
dopants are included, wherein the A-site dopants include one or
more of Pr and Ca and the B-site dopants include one or more of Cu,
Co, Zn, Fe and Ti.
5. The fuel cell of claim 3, wherein the mole fraction (1-x) of Mn
on the B-site is approximately 0.5 or greater.
6. The fuel cell of claim 2, wherein the Mn and Ni mixed B-site
compound comprises a
(La.sub.1-xSr.sub.x).sub.n+1(Ni.sub.1-xMn.sub.x).sub.nO.sub.3n+1
Ruddlesden-Popper compound.
7. The fuel cell of claim 1, wherein the reduced anode includes
between approximately 82 and approximately 95 wt % of the
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having the
Mn-based R-P phase constitution and between 5-18 wt % of the Ni
phase.
8. The fuel cell of claim 1, wherein the Ni phase constitution and
the (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having
a Mn-based R-P phase constitution, of n greater than or equal to 1,
is formed by adding an ionic phase including yttria and/or scandia
stabilized zirconia or rare-earth oxide stabilized ceria to the Ni
plus (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 Mn-based R-P
phases in an amount of 35-65 weight percent.
9. The fuel cell of claim 1, wherein the anode consists essentially
of the Ni phase constitution and the
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having the
Mn-based R-P phase constitution.
10. The fuel cell of claim 1, further comprising an anode
conductive layer adjacent the anode, wherein the anode conductive
layer comprises a cermet, where the metal phase comprises Ni.
11. The fuel cell of claim 10, wherein the metal phase is alloyed
with one or more of Pt, Pd, Cu, Co, Au, and Ag.
12. A method comprising forming a fuel cell, the fuel cell
including: a cathode; an electrolyte; and a reduced anode separated
from the cathode by the electrolyte, wherein the reduced anode
includes a Ni phase constitution and a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based Ruddlesden-Popper (R-P) phase constitution, wherein n is
equal to or greater than one, and wherein the anode, cathode, and
electrolyte are configured to form an electrochemical cell.
13. The method of claim 12, further comprising forming the reduced
anode via a reduction of a Mn and Ni mixed B-site compound having a
pervoskite structure or Ruddlesden-Popper compound that is present
following an initial anode processing step.
14. The method of claim 13, wherein the Mn and Ni mixed B-site
compound comprises a
(La.sub.1-xSr.sub.x)(Mn.sub.1-xNi.sub.x)O.sub.3 compound.
15. The method of claim 14, wherein additional A-site and B-site
dopants are included, wherein the A-site dopants include one or
more of Pr and Ca and the B-site dopants include one or more of Cu,
Co, Zn, Fe and Ti.
16. The method of claim 14, wherein the mole fraction (1-x) of Mn
on the B-site is approximately 0.5 or greater.
17. The method of claim 13, wherein the Mn and Ni mixed B-site
compound comprises a
(La.sub.1-xSr.sub.x).sub.n+1(Ni.sub.1-xMn.sub.x).sub.nO.sub.3n+1
Ruddlesden-Popper compound.
18. The method of claim 12, wherein the reduced anode includes
between approximately 82 and approximately 95 wt % of the
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having the
Mn-based R-P phase constitution and between 5-18 wt % of the Ni
phase.
19. The method of claim 12, wherein the Ni phase constitution and
the (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having
a Mn-based R-P phase constitution, of n greater than or equal to 1,
is formed by adding an ionic phase including yttria and/or scandia
stabilized zirconia or rare-earth oxide stabilized ceria to the Ni
plus (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 Mn-based R-P
phases in an amount of 35-65 weight percent.
20. The method of claim 12, wherein the anode consists essentially
of the Ni phase constitution and the
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having the
Mn-based R-P phase constitution.
21. The method of claim 12, further comprising an anode conductive
layer adjacent the anode, wherein the anode conductive layer
comprises a cermet, where the metal phase comprises Ni.
22. The method of claim 10, wherein the metal phase is alloyed with
one or more of Pt, Pd, Cu, Co, Au, and Ag.
23. A method of forming a fuel cell, the method comprising forming
an electrolyte on adjacent an as-processed anode, wherein the
electrolyte separates the as-processed anode from a cathode,
wherein the as-processed anode includes
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 or mixtures of
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 plus an ionic
phase, and wherein the anode, cathode, and electrolyte are
configured to form an electrochemical cell.
Description
TECHNICAL FIELD
[0002] The disclosure generally relates to fuel cells, such as
solid oxide fuel cells.
BACKGROUND
[0003] Fuel cells, fuel cell systems and interconnects for fuel
cells and fuel cell systems remain an area of interest. Some
existing systems have various shortcomings, drawbacks, and
disadvantages relative to certain applications. Accordingly, there
remains a need for further contributions in this area of
technology.
SUMMARY
[0004] Example compositions for anodes of fuels cells, such as,
e.g., solid oxide fuels cells, are described. For example, a
composition including a Ni phase constitution and Mn-based
Ruddlesden-Popper (R-P) phase constitution may be used to form an
anode for use in an electrochemical cell. When employed in a solid
oxide fuel cell, an anode of such a composition may display
relatively high durability in despite the relatively low oxygen
partial pressure operating environment of the fuel side of the
cell, e.g., due to the inherent thermodynamics of MnOx compounds
and the size of the Mn cations. Moreover, the presence of Ni phase
constitution dispersed within the R-P phase constitution may act as
a fuel oxidizing catalyst in combination with the catalytic
activity of the R-P phase constitution.
[0005] In one example, the disclosure is directed to a fuel cell
comprising a cathode; an electrolyte; and an reduced anode
separated from the cathode by the electrolyte, wherein the reduced
anode includes a Ni phase constitution and a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based Ruddlesden-Popper (R-P) phase constitution, wherein n is
equal to or greater than one, and wherein the anode, cathode, and
electrolyte are configured to form an electrochemical cell.
[0006] In another example, the disclosure is directed to a method
of making a fuel cell, the method comprising forming an electrolyte
on adjacent an anode, wherein the electrolyte separates the anode
from a cathode, wherein the anode includes a Ni phase constitution
and a (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound
having a Mn-based Ruddlesden-Popper (R-P) phase constitution,
wherein n is equal to or greater than one, and wherein the anode,
cathode, and electrolyte are configured to form an electrochemical
cell.
[0007] In one example, the Ni phase constitution and a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound can be
formed from the reduction of a
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 perovskite phase
present during the initial processing of the anode under high
oxygen partial pressure (air) firing conditions.
[0008] The details of one or more embodiments of the disclosure are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the disclosure will be
apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0009] The description herein makes reference to the accompanying
drawings wherein like reference numerals refer to like parts
throughout the several views.
[0010] FIG. 1 is a schematic diagram illustrating an example fuel
cell system in accordance with an embodiment of the present
disclosure.
[0011] FIG. 2 is a schematic diagram illustrating an example cross
section of a fuel cell system in accordance with an embodiment of
the present disclosure.
[0012] FIGS. 3-5 are plots illustrating properties of experimental
examples in accordance with embodiments of the present
disclosure.
[0013] Referring to the drawings, some aspects of a non-limiting
example of a fuel cell system in accordance with an embodiment of
the present disclosure is schematically depicted. In the drawing,
various features, components and interrelationships therebetween of
aspects of an embodiment of the present disclosure are depicted.
However, the present disclosure is not limited to the particular
embodiments presented and the components, features and
interrelationships therebetween as are illustrated in the drawings
and described herein.
DETAILED DESCRIPTION
[0014] As described above, examples of the present disclosure
relates to example compositions for anodes of fuels cells, such as,
e.g., solid oxide fuels cells. For example, a composition including
a Ni phase constitution and Mn-based R-P phase constitution may be
used to form an anode for use in an electrochemical cell. When
employed in a solid oxide fuel cell, an anode of such a composition
may display relatively high durability in despite the relatively
low oxygen partial pressure operating environment of the fuel side
of the cell, e.g., due to thermodynamic equilibrium characteristics
of MnOx compounds, and the size of the Mn cations are acceptable
for the formation of the R-P phase. Moreover, the presence of Ni
phase constitution dispersed within the R-P phase constitution may
act as a fuel oxidizing catalyst in combination with the catalytic
activity of the R-P phase constitution.
[0015] A variety of compositions may be used to form the various
components of a solid oxide fuel cell including anodes and
cathodes. In some examples, cathodes formed of nickelate based R-P
compounds may exhibit desirable electronic and catalytic
properties. For example, it has been shown that nickelate based R-P
compounds of the general formula Re.sub.n+1Ni.sub.nO.sub.3n+1 where
Re is an element of La, Pr, Nd or Sm or combinations thereof may be
used to form SOFC cathodes. Such compounds may have desirable mixed
ionic and electronic conductivity properties that result in low
cathode polarization resistances (e.g., relatively low area
specific resistance, ASR). However, such nickelate-based R-P
compounds may not be suitable in all cases as an anode material,
e.g., as the low oxygen partial pressure of the fuel side in a fuel
cell may result in phase decomposition of the anode material.
[0016] Despite the limitations surrounding the use of nickelate R-P
compounds, it has been determined that Mn-based nickelate R-P
compounds may be suitable to form an anode of a fuel cell despite
the low oxygen partial pressure of the fuel side in a fuel cell.
For example, based on the thermodynamic equilibrium of MnOx
compounds and the size of the Mn cations, Mn-based R-P compounds
may be favored to exist under the low oxygen partial pressure of
the fuel side in a fuel cell, and such an anode material may not be
susceptible to the phase decomposition occurring for anodes formed
from nickelate based R-P compounds. Whereas the nickel content of a
nickelate R-P will be fully reduced to metallic nickel under the
SOFC fuel environment, manganese content of oxide phase will tend
to be reduced to Mn2+ and thus available to maintain a stable oxide
phase under fuel environments. Accordingly, Mn-based R-P compounds
(e.g., of the formula
(La.sub.1-xSr.sub.x).sub.+1Mn.sub.nO.sub.3n+1), may be a suitable
material for a ceramic anode, e.g., as an alternative to Ni--YSZ
cermet based anodes, due to favorable mixed ionic and electronic
conductivity and microstructure retention under redox cycles and
overall durability of the ceramic. The order ("n" value) of the
Mn-based R-P phase will most commonly be the lower ordered
compounds (n=1 or 2) that require lower average valence states for
the Mn, as under the low partial pressure conditions of the fuel
environment, the valence state of Mn will tend towards Mn2+.
[0017] One challenge in realizing a phase stable Mn-based R-P
compound for an active anode (functioning in low oxygen partial
pressure) is that it must first be processed in an air environment
during the formation of the SOFC article, thus requiring a change
in the Mn valence state and therefore affecting the stoichiometry
and phase stability of the original oxide compound. In one example,
an intermediate oxide phase is utilized that is stable under the
standard air-firing conditions of SOFC, and that upon reduction
generates a correct stoichiometric Mn-based R-P compound that is
phase stable under the SOFC fuel environment based on achievement
of suitable Mn valence states and A-to-B site ratios as the degree
of A-site doping by cations such as Sr can influence the valence
state of the Mn on the B-site. One option to achieve the Mn-based
R-P phase under the fuel environment of the SOFC is to start with
an air-stable, Mn and Ni mixed B-site perovskites, such as, e.g.,
(La.sub.1-xSr.sub.x)(Mn.sub.1-x,Ni.sub.x)O.sub.3 anode formulation
during air firing. Upon reduction, these compounds change to a Ni
metal phase plus (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1,
which is a R-P compound that is most generally favored to exist
under low oxygen partial pressure where Mn valence trends toward
2+. As an additional benefit, a Ni phase is produced during
reduction that contributes catalytic properties in addition to the
Mn-based R-P phase.
[0018] FIG. 1 is a schematic diagram illustrating an example fuel
cell system 10 in accordance with an embodiment of the present
disclosure. As shown in FIG. 1, fuel cell system 10 includes a
plurality of electrochemical cells 12 (or "individual fuel cells")
formed on substrate 14. As will be described below, one or more of
the plurality of electrochemical cells 12 may include an anode
formed of the example compositions described herein.
Electrochemical cells 12 are coupled together in series by
interconnects 16. Fuel cell system 10 is a segmented-in-series
arrangement deposited on a flat porous ceramic tube, although it
will be understood that the present disclosure is equally
applicable to segmented-in-series arrangements on other substrates,
such on a circular porous ceramic tube. In various embodiments,
fuel cell system 10 may be an integrated planar fuel cell system or
a tubular fuel cell system.
[0019] Each electrochemical cell 12 includes an oxidant side 18 and
a fuel side 20. The oxidant is generally air, but could also be
pure oxygen (O.sub.2) or other oxidants, e.g., including dilute air
for fuel cell systems having air recycle loops, and is supplied to
electrochemical cells 12 from oxidant side 18. Substrate 14 may be
specifically engineered porosity, e.g., the porous ceramic material
is stable at fuel cell operation conditions and chemically
compatible with other fuel cell materials. In other embodiments,
substrate 14 may be a surface-modified material, e.g., a porous
ceramic material having a coating or other surface modification,
e.g., configured to prevent or reduce interaction between
electrochemical cell 12 layers and substrate 14. A fuel, such as a
reformed hydrocarbon fuel, e.g., synthesis gas, is supplied to
electrochemical cells 12 from fuel side 20 via channels (not shown)
in porous substrate 14. Although air and synthesis gas reformed
from a hydrocarbon fuel may be employed in some examples, it will
be understood that electrochemical cells using other oxidants and
fuels may be employed without departing from the scope of the
present disclosure, e.g., pure hydrogen and pure oxygen. In
addition, although fuel is supplied to electrochemical cells 12 via
substrate 14, it will be understood that in other embodiments, the
oxidant may be supplied to the electrochemical cells via a porous
substrate.
[0020] FIG. 2 is a schematic diagram illustrating an example cross
section of fuel cell system 10 in accordance with an embodiment of
the present disclosure. Fuel cell system 10 may be formed of a
plurality of layers screen printed onto substrate 14. This may
include a process whereby a woven mesh has openings through which
the fuel cell layers are deposited onto substrate 14. The openings
of the screen determine the length and width of the printed layers.
Screen mesh, wire diameter, ink solids loading and ink rheology may
determine the thickness of the printed layers. Fuel cell system 10
layers include an anode conductive layer 22, an anode layer 24, an
electrolyte layer 26, a cathode layer 28 and a cathode conductive
layer 30. In one form, electrolyte layer 26 may be a single layer
or may be formed of any number of sub-layers. It will be understood
that FIG. 2 is not necessarily to scale. For example, vertical
dimensions are exaggerated for purposes of clarity of
illustration.
[0021] In each electrochemical cell 12, anode conductive layer 22
conducts free electrons away from anode 24 and conducts the
electrons to cathode conductive layer 30 via interconnect 16.
Cathode conductive layer 30 conducts the electrons to cathode 28.
Interconnect 16 is embedded in electrolyte layer 26, and is
electrically coupled to anode conductive layer 22, and extends in
direction 32 from anode conductive layer 22 through electrolyte
layer 26, then in direction 36 from one electrochemical cell 12 to
the next adjacent electrochemical cell 12, and then in direction 32
again toward cathode conductive layer 30, to which interconnect 16
is electrically coupled. In particular, at least a portion of
interconnect 16 is embedded within an extended portion of
electrolyte layer 26, wherein the extended portion of electrolyte
layer 26 is a portion of electrolyte layer 26 that extends beyond
anode 24 and cathode 28, e.g., in direction 32, and is not
sandwiched between anode 24 and cathode 28.
[0022] Interconnects 16 for solid oxide fuel cells (SOFC) are
preferably electrically conductive in order to transport electrons
from one electrochemical cell to another; mechanically and
chemically stable under both oxidizing and reducing environments
during fuel cell operation; and nonporous, in order to prevent
diffusion of the fuel and/or oxidant through the interconnect. If
the interconnect is porous, fuel may diffuse to the oxidant side
and burn, resulting in local hot spots that may result in a
reduction of fuel cell life, e.g., due to degradation of materials
and mechanical failure, as well as reduced efficiency of the fuel
cell system. Similarly, the oxidant may diffuse to the fuel side,
resulting in burning of the fuel. Severe interconnect leakage may
significantly reduce the fuel utilization and performance of the
fuel cell, or cause catastrophic failure of fuel cells or
stacks.
[0023] For segmented-in-series cells, fuel cell components may be
formed by depositing thin films on a porous ceramic substrate,
e.g., substrate 14. In one form, the films are deposited via a
screen printing process, including the interconnect. In other
embodiments, other process may be employed to deposit or otherwise
form the thin films onto the substrate. The thickness of
interconnect layer may be 5 to 30 microns, but can also be much
thicker, e.g., 100 microns.
[0024] Interconnect 16 may be formed of a precious metal including
Ag, Pd, Au and/or Pt and/or alloys thereof, although other
materials may be employed without departing from the scope of the
present disclosure. For example, in other embodiments, it is
alternatively contemplated that other materials may be employed,
including precious metal alloys, such as Ag--Pd, Ag--Au, Ag--Pt,
Au--Pd, Au--Pt, Pt--Pd, Ag--Au--Pd, Ag--Au--Pt, Ag--Au--Pd--Pt
and/or binary, ternary, quaternary alloys in the Pt--Pd--Au-Ag
family, inclusive of alloys having minor non-precious metal
additions, cermets composed of a precious metal, precious metal
alloy, and an inert ceramic phase, such as alumina, or ceramic
phase with minimum ionic conductivity which will not create
significant parasitics, such as YSZ (yttria stabilized zirconia,
also known as yttria doped zirconia, yttria doping is 3-8 mol %,
preferably 3-5 mol %), ScSZ (scandia stabilized zirconia, scandia
doping is 4-10 mol %, preferably 4-6 mol %), doped ceria, and/or
conductive ceramics, such as conductive perovskites with A or
B-site substitutions or doping to achieve adequate phase stability
and/or sufficient conductivity as an interconnect, e.g., including
at least one of doped strontium titanate (such as
La.sub.xSr.sub.1-xTiO.sub.3-.delta., x=0.1 to 0.3), LSCM
(La.sub.1-xSr.sub.xCr.sub.1-yMn.sub.yO.sub.3, x=0.1 to 0.3 and
y=0.25 to 0.75), doped yttrium chromites (such as
Y.sub.1-xCa.sub.xCrO.sub.3-.delta., x=0.1-0.3) and/or other doped
lanthanum chromites (such as La.sub.1-xCa.sub.xCrO.sub.3-.delta.,
where x=0.15-0.3), and conductive ceramics, such as doped strontium
titanate, doped yttrium chromites, LSCM
(La.sub.1-xSr.sub.xCr.sub.1-yMn.sub.yO.sub.3), and other doped
lanthanum chromites. In one example, interconnect 16 may be formed
of y(PdxPt1-x)-(1-y)YSZ. Where x is from 0 to 1 in weight ratio,
preferably x is in the range of 0 to 0.5 for lower hydrogen flux. Y
is from 0.35 to 0.80 in volume ratio, preferably y is in the range
of 0.4 to 0.6.
[0025] Anode conductive layer 22 may be an electrode conductive
layer formed of a nickel cermet, such as such as Ni--YSZ (e.g.,
where yttria doping in zirconia is 3-8 mol %), Ni--ScSZ (e.g.,
where scandia doping is 4-10 mol %, preferably including a second
doping for example 1 mol % ceria for phase stability for 10 mol %
scandia-ZrO.sub.2) and/or Ni-doped ceria (such as Gd or Sm doping),
doped lanthanum chromite (such as Ca doping on A site and Zn doping
on B site), doped strontium titanate (such as La doping on A site
and Mn doping on B site),
La.sub.1-xSr.sub.xMn.sub.yCr.sub.1-yO.sub.3 and/or Mn-based R-P
phases of the general formula a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 Alternatively, it is
considered that other materials for anode conductive layer 22 may
be employed such as cermets based in part or whole on precious
metal. Precious metals in the cermet may include, for example, Pt,
Pd, Au, Ag, and/or alloys thereof. The ceramic phase may include,
for example, an inactive non-electrically conductive phase,
including, for example, YSZ, ScSZ and/or one or more other inactive
phases, e.g., having desired coefficients of thermal expansion
(CTE) in order to control the CTE of the layer to match the CTE of
the substrate and electrolyte. In some embodiments, the ceramic
phase may include Al.sub.2O.sub.3 and/or a spinel such as
NiAl.sub.2O.sub.4, MgAl.sub.2O.sub.4, MgCr.sub.2O.sub.4,
NiCr.sub.2O.sub.4. In other embodiments, the ceramic phase may be
electrically conductive, e.g., doped lanthanum chromite, doped
strontium titanate and/or one or more forms of LaSrMnCrO and/or R-P
phases of the general formula
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1
[0026] Electrolyte layer 26 may be made from a ceramic material. In
one form, a proton and/or oxygen ion conducting ceramic, may be
employed. In one form, electrolyte layer 26 is formed of YSZ, such
as 3YSZ and/or 8YSZ. In other embodiments, electrolyte layer 26 may
be formed of ScSZ, such as 4ScSZ, 6ScSz and/or 10Sc1CeSZ in
addition to or in place of YSZ. In other embodiments, other
materials may be employed. For example, it is alternatively
considered that electrolyte layer 26 may be made of doped ceria
and/or doped lanthanum gallate. In any event, electrolyte layer 26
is substantially impervious to diffusion therethrough of the fluids
used by fuel cell 10, e.g., synthesis gas or pure hydrogen as fuel,
as well as, e.g., air or O2 as an oxidant, but allows diffusion of
oxygen ions or protons.
[0027] Cathode layer 28 may be formed at least one of LSM
(La.sub.1-x Sr.sub.xMnO.sub.3, where x=0.1 to 0.3),
La.sub.1-xSr.sub.xFeO.sub.3, (such as where x=0.3),
La.sub.1-xSr.sub.xCo.sub.yFe.sub.1-yO.sub.3 (such as
La.sub.0.6Sr.sub.0.4Co.sub.0.2 Fe.sub.0.8O.sub.3) and/or
Pr.sub.1-xSr.sub.xMnO.sub.3 (such as
Pr.sub.0.8Sr.sub.0.2MnO.sub.3), although other materials may be
employed without departing from the scope of the present invention.
For example, it is alternatively considered that Ruddlesden-Popper
nickelates and La.sub.1-xCa.sub.xMnO.sub.3 (such as
La.sub.0.8Ca.sub.0.2MnO.sub.3) materials may be employed.
[0028] Cathode conductive layer 30 may be an electrode conductive
layer formed of a conductive ceramic, for example, at least one of
LaNi.sub.xFe.sub.1-xO.sub.3 (such as, e.g.,
LaNi.sub.0.6Fe.sub.0.4O.sub.3), La.sub.1-xSr.sub.xMnO.sub.3 (such
as La.sub.0.75Sr.sub.0.25MnO.sub.3), and/or
Pr.sub.1-xSr.sub.xCoO.sub.3, such as Pr.sub.0.8Sr.sub.0.2CoO.sub.3.
In other embodiments, cathode conductive layer 30 may be formed of
other materials, e.g., a precious metal cermet, although other
materials may be employed without departing from the scope of the
present invention. The precious metals in the precious metal cermet
may include, for example, Pt, Pd, Au, Ag and/or alloys thereof. The
ceramic phase may include, for example, YSZ, ScSZ and
Al.sub.2O.sub.3, or other non-conductive ceramic materials as
desired to control thermal expansion.
[0029] In accordance with one or more examples of the present
disclosure, anode 24 may include a Mn-based nickelate R-P phase
composition. In particular, anode 24 includes a Ni phase
constitution and a (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1
compound having a Mn-based R-P phase constitution, wherein n is
equal to or greater than zero. As noted above, based on MnOx
thermodynamics and the size of the Mn cations, Mn-based R-P
compounds may be favored to exist under the low oxygen partial
pressure of the fuel side in a fuel cell. An example range of
oxygen partial pressures throughout the operation of a solid oxide
fuel cell are from 10.sup.-17 atm (stack outlet fuel composition)
to 10.sup.-20 atm representative of an anode protection gas mixture
of H.sub.2/N.sub.2 containing some H.sub.2O as a result of
parasitic currents. Such an anode material may not be susceptible
to the phase decomposition occurring for anodes formed from
La-based nickelate R-P compounds, while still exhibiting desirable
electronic and catalytic properties. Further, the presence of Ni
phase constitution dispersed within the R-P phase constitution may
act as a fuel oxidizing catalyst in combination with the catalytic
activity of the R-P phase constitution.
[0030] The nickelate R-P composition of anode 24 may be formed
using any suitable technique. In some examples, to form the
Mn-based nickelate R-P composition of anode 24, the initial
starting powder is a Mn and Ni mixed B-site perovskites, such as,
e.g., (La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 may be
utilized during air firing in the initial processing step of
applying the anode precursor within the fuel cell structure. In
some case, mixed B-site perovskites of the general formula
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 may be used. Upon
reduction in a fuel environment, these compounds change to a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1, where n is equal to
or greater than one, which is a R-P compound plus residual Ni metal
phase. As noted herein, the finely dispersed Ni phase may act as a
fuel oxidizing catalyst in combination with the catalytic activity
of the Mn-based R-P phase. Both the Mn-based R-P phase and Ni
phases may contribute to the electronic conductivity needed for an
active anode.
[0031] Various other A-site and B-site cations may be included in
the initial perovskite starting powder, for instance Ca and Pr for
the A-site and various B-site dopants such as Cu and Co that may be
minimally retained in the R-P structure on reduction, but which may
most preferentially be alloyed with the Ni alloy phase. Additional
Ni content can be incorporated into the resulting anode by the
addition of NiO with the
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 powder within the
processing of the anode into the fuel cell structure.
[0032] There is the potential to in-situ form a Mn plus Ni mixed
B-site R-P compound during initial anode firing by mixing of
(La.sub.1-xSr.sub.x)MnO.sub.3 and R-P nickelate,
(La.sub.1-xSr.sub.x).sub.n+1Ni.sub.nO.sub.3n+1, by selecting the
correct molar ratios of A-site and B-site cations in the two
powders and the correct molar ratio of the two powders. Upon
reduction a Mn-based R-P phase constitution in addition to a
dispersed Ni phase can result. For example:
2(La.sub.1-xSr.sub.x)(Mn)O.sub.3+(La.sub.1-xSr.sub.x).sub.2NiO.sub.4.fwd-
arw.(La.sub.1-xSr.sub.x).sub.4(Mn.sub.2Ni)O.sub.10 (air)
Ni+2(La.sub.1-xSr.sub.x).sub.2MnO.sub.4(reduced)
[0033] Where higher ordered (e.g., n=2,3) Mn-based R-P phases may
be able to be formed in air because of the higher required B-site
valence state in such compounds, and where these compounds could be
mixed with NiO such that there could be an in-situ reaction during
anode processing forming a mixed Ni and Mn B-site higher ordered
R-P compound stable in air. With careful selection of chemistry and
stoichiometry, upon reduction a Mn-based R-P phase constitution in
addition to a dispersed Ni phase can result. Alternatively, the
mixed Ni and Mn B-site higher ordered R-P compound could be the
actual starting composition of the powder rather than formed
in-situ. For example:
Ex.
1/3NiO+4/3(La.sub.1-xSr.sub.x).sub.3Mn.sub.2O.sub.7.fwdarw.(La.sub.1-
-xSr.sub.x).sub.4(Mn.sub.2.67Ni.sub.0.33)O.sub.10(air)
Ni+(La.sub.1-xSr.sub.x).sub.3Mn.sub.2O.sub.4(reduced)
[0034] Other combinations of starting compounds are possible with
the end objective of achieving a Mn-based R-P phase constitution in
addition to a dispersed Ni phase upon anode reduction. Limitations
on processing routes may exist based on the inability to achieve
stable mixed B-site Mn--Ni R-P phases stable in air, but these
examples are illustrative as potential routes to forming the
desired anode phases.
[0035] The composition of anode 24 may be such that substantially
all of anode 24 is formed of a
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based R-P phase constitution in addition to Ni phase dispersed
in the (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound. For
example, anode 24 may include at least 80 wt % of the
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based R-P phase constitution, such as, e.g., at least 82 wt %
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound having a
Mn-based Ruddlesden-Popper (R-P) phase constitution. An example is
(La.sub.0.75Sr.sub.0.25)(Mn.sub.0.5Ni.sub.0.5)O.sub.3 which upon
reduction leads to 83.1% (La.sub.0.75Sr.sub.0.25).sub.2MnO.sub.4
and 16.9% Ni.
[0036] The nickel phase may be present along with the
(La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound in an
amount that allows for the Ni to act as a fuel oxidizing catalyst
in combination with the catalytic activity of the R-P phase
constitution. For example, anode 24 may include approximately 5 to
approximately 18 percent by weight, As an example,
(La.sub.0.75Sr.sub.0.25)(Mn.sub.0.5Ni.sub.0.5)O.sub.3 which upon
reduction leads to 83.1% (La.sub.0.75Sr.sub.0.25).sub.2MnO.sub.4
and 16.9% Ni at one end of the range (n=1 order) and
(La.sub.0.75Sr.sub.0.25)(Mn.sub.0.75Ni.sub.0.25)O.sub.3 which upon
reduction leads to 93.2%
(La.sub.0.75Sr.sub.0.25).sub.4Mn.sub.3O.sub.4 and 6.8% Ni (for n=3
order), and at higher order R-P compounds the Ni content would be
less. The prior examples are indicative of Ni contents that could
be achieve from dissolution from the perovskite phase upon
reduction, but additional Ni content may be achieved by including
NiO content along with the
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3 during the initial
processing of the anode.
[0037] Although it may be desirable to achieve phase pure B-site
mixed perovskite or R-P phases during the initial air-sintering of
the anode and likewise upon reduction to achieve only the desired
R-P phase plus Ni constitution, it is realized that some secondary
phases from incomplete reactions during powder synthesis or in-situ
formulation may result and that some minor phases other than the
R-P and Ni phases may occur upon reduction because of incomplete
phase decomposition, however, the presence of minor amounts of
these phases may still provide for favorable performance and
durability of the solid oxide fuel cell.
[0038] In cases in which the anode composition is formed via the
reduction of Mn and Ni mixed B-site perovskites, the amount of Ni
phase dispersed in the Mn-based R-P may depend on the composition
of the Mn and Ni mixed B-site perovskites. In particular, the ratio
of the Mn to Ni content on the B-site of the perovskite composition
will dictate the nature of the resulting Mn-based R-P compound. As
an example, the below table illustrate theoretical composition
resulting from the reduction of Mn and Ni mixed B-site
perovskites.
TABLE-US-00001 Mn fraction (y) on B-site 0.5 0.67 0.75 Theoreti-
(La.sub.1-xSr.sub.x).sub.2MnO.sub.3
(La.sub.1-xSr.sub.x).sub.3Mn.sub.2O.sub.7
(La.sub.1-xSr.sub.x).sub.4Mn.sub.3O.sub.10 cal re- sulting Mn R-P
com- pound on reduction Order of 1 2 3 R-P phase
[0039] The perovskite composition does not necessarily need to be
at 0.5. 0.67 and 0.75. Values for the Mn fraction on the B-site
different from 0.5. 0.67 and 0.75 would be expected to yield
combinations of different ordered R-P compositions. The relative
levels of free metallic Ni in the reduced anode would be greatest
for perovskite Mn volume fractions at about 0.5. As there could be
some remaining solubility of Ni within the ceramic phase upon
reduction, the resulting R-P phase may have some mixed Mn and Ni on
the B-site, there could be some remaining perovskite phase present
and the final metallic Ni phase would thus be less than theoretical
phases along with any residual perovskite phases.
[0040] In some examples, the composition of anode 24 may include
one or more additives, elements, or compounds other than Ni phase
dispersed in the (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1
compound having a Mn-based R-P phase constitution. In one example,
the Ni plus (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 Mn-based
R-P phases may be composited with an ionic phase such as 8YSZ, or
yttria stabilized zirconias (YSZ) of 3-8 mole percent yttria,
scandia stabilized zirconia (ScSZ) of 4-10 mole percent scandia and
may include minor additional stabilizers such as 1 mole percent Ce
or 0.5-1 mole percent alumina, and may include doped ceria where
the dopant is one or more of Gd, Y, Sm and/or Pr. In such an
embodiment, the starting mixture for the anode consists of the
(La.sub.1-xSr.sub.x)(Mn.sub.yNi.sub.1-y)O.sub.3, the ionic phase
and could include additional NiO content as desired to impact
polarization resistance and bulk electronic conductivity of the
reduced anode. In one example, anode 24 may consist essentially of
Ni and the (La.sub.1-xSr.sub.x).sub.n+1Mn.sub.nO.sub.3n+1 compound
having a Mn-based R-P phase constitution, where the additionally
material in present only in an amount that does not alter one or
more properties of the material in a manner that does not allow
anode 24 to function as described herein.
[0041] As noted above, anode conductive layer 22 may be an
electrode conductive layer formed of a nickel cermet. Since in some
examples, predominately ceramic-based anodes, including those
described herein, may provide for a relatively low polarization
ASR, however the electronic conductivity in some instances may be
insufficient. Accordingly, anode conductive layer 22 may be a
Ni-based cermet composition to account for the low electronic
conductivity of anode 24.
[0042] Any suitable technique may be employed to form
electrochemical cell 12 of FIGS. 1 and 2. In the example of FIG. 2,
anode conductive layer 22 may be printed directly onto substrate
14, as is a portion of electrolyte 26. Anode layer 24 may be
printed onto anode conductive layer 22. Portions of electrolyte
layer 26 may be printed onto anode layer 24, and portions of
electrolyte layer 26 are printed onto anode conductive layer 22 and
onto substrate 14. Cathode layer 28 is printed on top of
electrolyte layer 26. Portions of cathode conductive layer 30 are
printed onto cathode layer 28 and onto electrolyte layer 26.
Cathode layer 28 is spaced apart from anode layer 24 in a direction
32 by the local thickness of electrolyte layer 26.
[0043] Anode layer 24 includes anode gaps 34, which extend in a
direction 36. Cathode layer 28 includes cathode gaps 38, which also
extend in direction 36. In the example of FIG. 2, direction 36 is
substantially perpendicular to direction 32, although the present
disclosure is not so limited. Gaps 34 separate anode layer 24 into
a plurality of individual anodes 40, one for each electrochemical
cell 12. Gaps 38 separate cathode layer 28 into a corresponding
plurality of cathodes 42. Each anode 40 and the corresponding
cathode 42 that is spaced apart in direction 32 therefrom, in
conjunction with the portion of electrolyte layer 26 disposed
therebetween, form an electrochemical cell 12.
[0044] Similarly, anode conductive layer 22 and cathode conductive
layer 30 have respective gaps 44 and 46 separating anode conductive
layer 22 and cathode conductive layer 30 into a plurality of
respective anode conductor films 48 and cathode conductor films 50.
The terms, "anode conductive layer" and "anode conductor film" may
be used interchangeably, in as much as the latter is formed from
one or more layers of the former; and the terms, "cathode
conductive layer" and "cathode conductor film" may be used
interchangeably, in as much as the latter is formed from one or
more layers of the former.
[0045] In some examples, anode conductive layer 22 has a thickness,
i.e., as measured in direction 32, of approximately 5-15 microns,
although other values may be employed without departing from the
scope of the present disclosure. For example, it is considered that
in other embodiments, the anode conductive layer may have a
thickness in the range of approximately 5-50 microns. In yet other
embodiments, different thicknesses may be used, e.g., depending
upon the particular material and application.
[0046] Similarly, anode layer 24 may have a thickness, i.e., as
measured in direction 32, of approximately 5-20 microns, although
other values may be employed without departing from the scope of
the present invention. For example, it is considered that in other
embodiments, the anode layer may have a thickness in the range of
approximately 5-40 microns. In yet other embodiments, different
thicknesses may be used, e.g., depending upon the particular anode
material and application.
[0047] Electrolyte layer 26 may have a thickness of approximately
5-15 microns with individual sub-layer thicknesses of approximately
5 microns minimum, although other thickness values may be employed
without departing from the scope of the present invention. For
example, it is considered that in other embodiments, the
electrolyte layer may have a thickness in the range of
approximately 5-40 microns. In yet other embodiments, different
thicknesses may be used, e.g., depending upon the particular
materials and application.
[0048] Cathode layer 28 has a thickness, i.e., as measured in
direction 32, of approximately 10-20 microns, although other values
may be employed without departing from the scope of the present
invention. For example, it is considered that in other embodiments,
the cathode layer may have a thickness in the range of
approximately 10-50 microns. In yet other embodiments, different
thicknesses may be used, e.g., depending upon the particular
cathode material and application.
[0049] Cathode conductive layer 30 has a thickness, i.e., as
measured in direction 32, of approximately 5-100 microns, although
other values may be employed without departing from the scope of
the present invention. For example, it is considered that in other
embodiments, the cathode conductive layer may have a thickness less
than or greater than the range of approximately 5-100 microns. In
yet other embodiments, different thicknesses may be used, e.g.,
depending upon the particular cathode conductive layer material and
application.
[0050] Although not shown in FIG. 2, in some examples, fuel cell
system 10 may include one or more chemical barrier layers between
interconnect 16 and adjacent components to reduce or prevent
diffusion between the interconnect and adjacent components, e.g.,
an anode and/or an anode conductor film and/or cathode and/or
cathode conductor film, may adversely affect the performance of
certain fuel cell systems. In various embodiments, such a chemical
barrier layer may be configured to prevent or reduce material
migration or diffusion at the interface between the interconnect
and an anode, and/or between the interconnect and an anode
conductor film, and/or between the interconnect and a cathode,
and/or between the interconnect and a cathode conductor film which
may improve the long term durability of the interconnect. For
example, without a chemical barrier, material migration (diffusion)
may take place at the interface between an interconnect formed of a
precious metal cermet, and an anode conductor film and/or anode
formed of a Ni-based cermet. The material migration may take place
in both directions, e.g., Ni migrating from the anode conductive
layer/conductor film and/or anode into the interconnect, and
precious metal migrating from the interconnect into the conductive
layer/conductor film and/or anode. The material migration may
result in increased porosity at or near the interface between the
interconnect and the anode conductor film and/or anode, and may
result in the enrichment of one or more non or low-electronic
conducting phases at the interface, yielding a higher area specific
resistance (ASR), and hence resulting in reduced fuel cell
performance. Material migration between the interconnect and the
cathode and/or between the interconnect and the cathode conductor
film may also or alternatively result in deleterious effects on
fuel cell performance. Such a chemical barrier layer may be formed
of one or both of two classes of materials; cermet and/or
conductive ceramic.
EXAMPLES
[0051] Various experiments were carried out to evaluate one or more
aspects of example anode compositions in accordance with the
disclosure. However, examples of the disclosure are not limited to
the experimental anode compositions.
[0052] Three different Mn+Ni B-site perovskite powders where
obtained from TransTech, Inc. (Adamstown, Md.). In particular, the
first example powder obtained was
(La.sub.0.875Sr.sub.0.125)(Mn.sub.0.5Ni.sub.0.5)O.sub.3, which was
designed to form (La.sub.0.875Sr.sub.0.125).sub.2MnO.sub.4 n=1
ordered R-P phase constitution plus Ni metal phase constitution on
reduction. After reduction, the composition was analyzed via X-ray
diffraction. The X-ray diffraction showed some R-P phase,
perovskite phase, metallic Ni phase and some free La.sub.2O.sub.3.
Based on the La.sub.2O.sub.3 content, it was determined that the
example composition was not favorable for the desired phase
generation.
[0053] The second example powder obtained for evaluation was
(La.sub.0.67Sr.sub.0.33).sub.0.97(Mn.sub.0.67Ni.sub.0.33)O.sub.3,
which was designed to form
(La.sub.0.67Sr.sub.0.33).sub.2.91Mn.sub.2O.sub.7 n=2 ordered R-P
phase constitution (with slight A-site deficiency) plus Ni metal
phase constitution on reduction. After reduction, the composition
was analyzed via X-ray diffraction. The X-ray diffraction showed
the R-P phase and Ni metal phase. FIG. 3 is a plot illustrating
electrical conductivity measurements taken for
(La.sub.0.67Sr.sub.0.33).sub.0.97(Mn.sub.0.67Ni.sub.0.33)O.sub.3 in
both air and a low oxygen partial pressure (1.times.10.sup.-18
atm). As illustrated, the electrical conductivity measurements
showed conductivity in low pO.sub.2 fuel of 3-4 S/cm at typical
fuel cell operating temperatures.
[0054] The third example powder obtained for evaluation was
(La.sub.0.45Sr.sub.0.55).sub.0.97(Mn.sub.0.5Ni.sub.0.5)O.sub.3,
which was designed to form
(La.sub.0.45Sr.sub.0.55).sub.1.94MnO.sub.4 n=1 ordered R-P phase
constitution (with slight A-site deficiency) plus Ni metal phase
constitution on reduction. After reduction, the composition was
analyzed via X-ray diffraction. The X-ray diffraction showed the
R-P phase, likely some minor residual perovskite phase, and Ni
metal phase. FIG. 4 is a plot illustrating the results of the X-ray
diffraction analysis of the reduced third example composition. As
shown, the results indicate that Ni metal phase is present in the
reduced composition (other lines are for R-P phase and there is
very minor MnO present).
[0055] Three individual electrochemical fuel cells were fabricated
using the second and third example powder compositions to form
anodes for the cells. For these cells the cathode was an LSM+YSZ
composite, the cathode current collector was 100% LSM of the same
composition of the LSM in the cathode, the anode current collector
was a NiPd alloy cermet with YSZ and MgAl.sub.2O.sub.4 ceramic
phases. In particular, the active anodes for each of the three
cells were as follows:
[0056]
(La.sub.0.45Sr.sub.0.55).sub.0.97(Mn.sub.0.5Ni.sub.0.5)O.sub.3
(referred to as B2:LSMN50).
[0057]
(La.sub.0.67Sr.sub.0.33).sub.0.97(Mn.sub.0.67Ni.sub.0.33)O.sub.3
plus 10Sc1CeSZ ionic phase (referred to as B1:LSMN67+10Sc)
[0058]
(La.sub.0.45Sr.sub.0.55).sub.0.97(Mn.sub.0.5Ni.sub.0.5)O.sub.3 plus
10Sc1CeSZ ionic phase (referred to as A2:LSMN50+10Sc).
[0059] FIG. 5 is a plot illustrating measured voltage versus
current density for each of the three cells fabricated for
evaluation. As shown, best performing cell was included
B1:LSMN67+10Sc as the active anode with ASR in the 0.6 ohm-cm.sup.2
range. Even though the ASR value is higher than traditional Ni--YSZ
based anodes, the performance expects to be further improved
through optimizing composition and microstructure. These result
showed improved performance when compositing the Mn-based R-P and
Ni phases with an ionic phase
[0060] Various embodiments of the invention have been described.
These and other embodiments are within the scope of the following
claims.
* * * * *